1. Field of the Invention
The invention relates to a process and apparatus for surface coating of metals.
2. Description of the Prior Art
It is well known that surface depositions or coatings of various steel tools act as protective barriers against atmospheric effects, such as oxidation, as well as increasing their hardness and durability. The coatings applied to steel and carbide tools include titanium nitride, silicon nitride, titanium carbide, aluminum oxide and various combinations of the above. The addition of these coatings increase tool life from 2 to 5 times. The coatings applied are from 0.0002 to 0.002 in. thick depending upon the tooling requirements.
One of the commercial coating processes is called chemical vapor deposition (CVD) and it is used to treat both carbide and steel tools. Coatings are thin, hard, and have lubrication properties that improve the cutting action and reduce the coefficient of friction of tooling. Another commercial coating process is called plasma vapor deposition or ion implant coating (PVD).
In general, both of these processes involve applying the coating by loading the tools to be coated inside a vacuum chamber, evacuating the chamber with the parts up to some suitable temperature, in the case of CVD coatings from 1750.degree. F. to 1950.degree. F. and then introducing mixtures of suitable gases such as titanium chloride, ammonia and nitrogen. The chemical reaction at the high temperatures causes a deposit of titanium nitride or other hard coating to form on the surface of the tools over a period of two to four hours. Following the reaction, the vessel is cooled and the tools removed.
In the case of PVD coatings, a vacuum chamber with elaborate electrical controls for the sputtering is necessary, and though various parts can be coated with any combination of coatings, the coating is deposited only in a line of sight direction, and the parts must be rotated to achieve uniform coating.
In both of these commercially available processes, very costly vacuum systems are necessary, with attendant high maintenance costs typical of these systems. Also, treatment cycles are long due to the requirement to heat up, evacuate and cooldown the system. In addition, these systems are limited in the size that they can be constructed and remain technically feasible.
In the past few years, commercial atmosphere fluid bed heat treating has been brought to successful commercial use in heat treating. The use of fluid bed furnaces in various through and surface hardening treatments such as neutral hardening, nitriding, carburizing and nitrocarburizing has been found to offer significant advantage over alternative processes. In specific cases where case hardening is the treatment, it has been found that extreme uniformity can be obtained in a fluid bed furnace. In addition, there are operating cost advantages and conveniences to using a fluid bed furnace. In addition, fluid bed furnaces of very large volume have been built and deminstrated to be feasible to operate.
The apparatus and method used for surface coating in the present invention provides a number of the above mentioned advantages in terms of uniformity, low operating costs, and larger scale operation.